To determine the blue book value of this Fifth Wheel, you will need to reference Flagstaff by Forest River with NADA. Note, the misspelling of [Forrest] in your original question has been corrected.
During 2013, the model was named the [Classic Super Lite Series]. There were 10 models ranging from 26 to 31 feet long. All are 8 feet wide. To find the value, you will need to reference the specific model.
Using the 26 foot M-8524RLWS model of the Classic Super Lite Flagstaff by Forest River as an example, the base value is:
Low Retail - $11800
High Retail - $14200
Additional options including air conditioning/heating and appliances would increase this base bluebook value. The supply and demand in your location can also influence the used price of any RV.